- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Possible scenario:Since the OP says that it pumped water for awhile (but not whether it was at full flow) the "locked" rotor is probably just a lot of extra drag.
What is confusing me is that after one fuse blows the current in the remaining two fuses is not high enough to blow one of them (and the pump continues to move water?).
My brain is not coming up with anything that would cause a momentary or short term overcurrent but allow the pump to continue running for minutes until the overload trips.
Maybe something about the torque/thrust that is reduced when the first fuse blows, reducing the resistance to rotation, hence reducing the current seen by the remaining pair of fuses?
Fuse blows, motor single phases, torque drops to roughly 1/2 of normal, which drops flow and load on the motor so it has less up thrust against the thrust bearing problem. So it continues to run and move SOME water, but not what it's supposed to be pumping. The reduced load allows the other fuses to hold in, but the current imbalance is being picked up by the Overload Relay (luckily).
This is why I hate fuses on centrifugal pump controls... If you use them, you MUST use a good phase loss protection scheme. I prefer using Solid State OL relays now, because most of them do current based phase loss protection, which is better than voltage base line monitor relays. A typical SSOL would take the motor off line within 3 seconds of having no current on one phase.